The present invention generally relates to power tools, and more particularly, to a decoupling mechanism that can be used as part of safety detection and protection systems for power tools such as circular saws of the type that are used for cutting wood and other materials.
For as long as power tools have existed, there has been concern for the safety of those who operate them. This is particularly true with regard to power tools of the type which have relatively large exposed moving blades that can easily cause serious injury to individuals who carelessly use the tool or who are the victim of a true accident that results from unforeseen conditions. While safety systems have been developed for machine tools and other commercial tools, such as stamping tools, punch presses and other machines which exert great force in manufacturing metal and other parts, such systems often have a level of sophistication and complexity that is achieved only at a cost that is prohibitive if considered for use in tools that are marketed to individual consumers or small contractors and tradesmen.
More particularly, the well known circular saw that is used by woodworkers and tradesmen has a rotating blade that can be exposed during use even though blade guards have been provided with such saws when sold for the last several decades. Such blade guards, while effective to prevent some injuries, are considered unwieldy by many operators. In fact, many operators find that such blade guards actually interfere with the use of the circular saws and therefore remove such blade guards most of the time. Obviously, a rotating circular saw blade can cause great injury to an operator and countless hand injuries occur annually because of careless use or the occurrence of fortuitous accidents.
Safety systems have been developed for use on saws to stop the blade when the operator's hand approaches the blade and which trigger a brake mechanism that typically includes a pawl structure that is urged into the blade by a biasing mechanism that is retained by a fusible member that is melted by a high energy electrical current. Severing the fusible member releases the pawl to stop the blade, or in some embodiments causes the blade to be retracted down below the work surface in the case of the system being utilized with a table saw. All of these systems have one or more disadvantages in terms of cost, convenience of use, early and effective detection of a dangerous condition and the ability to provide graduated protective actions as a function of the characteristics that develop during a potentially dangerous condition. Further, many protection systems are destructive to the blade to the extent that the blade must be replaced after a braking event.
The blade is driven by the drive shaft, which is in turn driven by the motor. In some protection systems, the blade is decelerated or stopped upon sensing a dangerous condition. When enough pressure or torque is supplied to the saw blade to decelerate or stop the blade, a force is exerted on the drive shaft to slow or stop with the blade. However, the drive shaft continues to be driven with the torque provided by the motor. Since the drive shaft is being acted upon by both the motor on one end, and the pressure/torque applied to the blade on the other end, the saw is vulnerable to failure. For this reason, a decoupling mechanism is needed to decouple the motor from the blade.